Duocarmycin SA (Compound 1) [Ichimura et al., J. Antibiot. 1990, 43:1037-1038] and CC-1065 (Compound 2) [Martin et al., J. Antibiot. 1981, 34:1119-1125], below, are the two most widely
recognized members of a class of exceptionally potent naturally occurring antitumor agents that also include duocarmycin A [Takahashi et al., J. Antibiot. 1988, 41:1915-1917], yatakemycin [Igarashi et al., J. Antibiot. 2003, 56:107-113](below).
This unique class of natural products derives its antitumor properties from their ability to alkylate DNA in a sequence selective manner. [For duocarmycin SA, see: (a) Boger et al., J. Am. Chem. Soc. 1994, 116:1635-1656; for yatakemycin, see: (b) Parrish et al., J. Am. Chem. Soc. 2003, 125:10971-10976; (c) Trzupek et al., Nat. Chem. Biol. 2006, 2:79-82; (d) Tichenor et al., J. Am. Chem. Soc. 2007, 129:10858-10869; for CC-1065, see: (e) Hurley et al., Biochemistry 1988, 27:3886-3892; (f) Boger et al., Bioorg. Med. Chem. 1994, 2:115-135; (g) Boger et al., J. Am. Chem. Soc. 1991, 113:3980-3983; (h) Boger et al., Proc. Nat. Acad. Sci. U.S.A. 1991, 88:1431-1435; (i) Boger et al., J. Am. Chem. Soc. 1990, 112:4623-4632; (j) Boger et al., J. Am. Chem. Soc. 1991, 113:3980-3983; for duocarmycin A, see: (k) Boger et al., J. Am. Chem. Soc. 1990, 112:8961-8971; (l) Boger et al., J. Am. Chem. Soc. 1991, 113:6645-6649; (m) Boger et al., Med. Chem. Lett. 1992, 2:759-765. (n) Boger et al., J. Am. Chem. Soc. 1993, 115:9872-9873; (o) Boger et al., Chem.-Biol. Interactions 1990, 73:29-52. Reviews: (a) Boger et al., Angew. Chem., Int. Ed. Engl. 1996, 35:1438-1474; (b) Boger et al., Acc. Chem. Res. 1995, 28:20-29; (c) Boger et al., Proc. Natl. Acad. Sci. U.S.A. 1995, 92:3642-3649; (d) Boger et al., Acc. Chem. Res. 1999, 32:1043-1052; (e) Tichenor et al., Natural Prod. Rep. 2008, 25:220-226; (f) MacMillan et al., J. Med. Chem. 2009, 52:5771-5780; (g) Searcey et al., Curr. Pharm. Des. 2002, 8:1375-1389; and (h) Tse et al., Chem. Biol. 2004, 11:1607-1617.]
In depth studies of the natural products, their synthetic unnatural enantiomers, [(a) Boger et al., J. Am. Chem. Soc. 1988, 110:1321-1323; (b) Boger et al., J. Am. Chem. Soc. 1988, 110:4796-4807; (c) Boger et al., J. Am. Chem. Soc. 1992, 114:10056-10058; (d) Boger et al., J. Am. Chem. Soc. 1993, 115:925-9036. (e) Boger et al., J. Am. Chem. Soc. 1996, 118:2301-2302; (f) Boger et al., J. Am. Chem. Soc. 1997, 119:311-325; (g) Boger et al., Chem. Rev. 1997, 97:787-828; (h) Tichenor et al., J. Am. Chem. Soc. 2004, 126:8396-8398; (i) Tichenor et al., J. Am. Chem. Soc. 2006, 128:15683-15696; (j) MacMillan et al., J. Am. Chem. Soc. 2009, 131:1187-1194.] and key analogues have defined many of the fundamental features that control their DNA alkylation selectivity, efficiency, and catalysis, resulting in a detailed understanding of the relationships between structure, reactivity, and biological activity. [See the citations above and (a) Boger et al., J. Am. Chem. Soc. 1997, 119:4977-4986; (b) Boger et al., J. Am. Chem. Soc. 1997, 119:4987-4998; (c) Boger et al., Bioorg. Med. Chem. 1997, 5:263-276.]
CBI (1,2,9,9a-tetrahydrocyclopropa[c]benz-[e]indol-4-one), below, is one of the most
studied synthetic analogues of the duocarmycin family since the inventor and co-workers first introduced it in 1989. [(a) Boger et al., J. Am. Chem. Soc. 1989, 111:6461-6463; (b) Boger et al., J. Org. Chem. 1990, 55:5823-5832; (c) Boger et al., Tetrahedron Lett. 1990, 31:793-796; (d) Boger et al., Bioorg. Med. Chem. Lett. 1991, 1:55-58; (e) Boger et al., J. Am. Chem. Soc. 1992, 114:5487-5496; (f) Boger et al., J. Am. Chem. Soc. 1994, 116:5523-5524; (g) Boger et al., Bioorg. Med. Chem. 1995, 3:761-775; (h) Boger et al., J. Am. Chem. Soc. 1994, 116:7996-8006; (i) Parrish et al., Bioorg. Med. Chem. 2003, 11:3815-3838; (j) Boger et al., J. Am. Chem. Soc. 1990, 112:5230-5240.]
The CBI alkylation subunit is not only synthetically more accessible and possesses DNA alkylation properties identical to those of the natural products, [(a) Boger et al., J. Org. Chem. 1992, 57:2873-2876; (b) Boger et al., J. Org. Chem. 1995, 60:1271-1275; (c) Boger et al., Synlett 1997, 515-517; (d) Kastrinsky et al., J. Org. Chem. 2004, 69:2284-2289; (e) Lajiness et al., J. Org. Chem. 2010, 76:583-587; (f) Drost et al., J. Org. Chem. 1991, 56:2240-2244; (g) Aristoff et al., J. Org. Chem. 1992, 57:6234-6239; (h) Mohamadi et al., J. Med. Chem. 1994, 37:232-239; and (i) Ling et al., Heterocycl. Commun. 1997, 3:405-408], but it is also four times more stable and four times more potent than the naturally occurring alkylation subunit of CC-1065 (Compound 2) below, approaching the stability
and potency of the duocarmycin SA (Compound 1) alkylation subunit, below. Because the CBI-based
analogues have also been established to exhibit efficacious in vivo antitumor activity in animal models, it has become an excellent synthetic replacement on which to examine the structure-function features of the natural products, including new pro-drug designs. [(a) Boger et al., Bioorg. Med. Chem. 1995, 3:1429-1453; and (b) Boger et al., Bioorg. Med. Chem. Lett. 1991, 1:115-120.]
In the course of the early total syntheses [(a) Boger et al., J. Am. Chem. Soc. 1988, 110:1321-1323; (b) Boger et al., J. Am. Chem. Soc. 1988, 110:4796-4807; (c) Boger et al., J. Am. Chem. Soc. 1992, 124:10056-10058; (d) Boger et al., J. Am. Chem. Soc. 1993, 115:9025-9036; (e) Boger et al., J. Am. Chem. Soc. 1996, 118:2301-2302; (f) Boger et al., J. Am. Chem. Soc. 1997, 119:311-325; (g) Boger et al., Chem. Rev. 1997, 97; 787-828; (h) Tichenor, M. S.; Kastrinsky et al., J. Am. Chem. Soc. 2004, 126; 8396-8398; (i) Tichenor et al., J. Am. Chem. Soc. 2006, 128:15683-15696; (j) MacMillan et al., J. Am. Chem. Soc. 2009, 131:1187-1194] (noted above) of the natural products and related analogues including CBI-indole2 (Compound 5) below [(a) Boger et al., Bioorg.
Med. Chem. 1995, 3:1429-1453; and (b) Boger et al., Bioorg. Med. Chem. Lett. 1991, 1:115-120], synthetic phenol precursors such as Compound 4, below, which have yet to undergo the Winstein Ar-3′ spiro-
cyclization, were found to be equipotent to and indistinguishable from their cyclized cyclopropane containing counterparts in cell growth inhibition assays, DNA alkylation studies, and in vivo antitumor models.
Due to this indistinguishable behavior, protection of the phenol provides an especially effective site on which to prepare inactive pro-drugs that can be cleaved in vivo releasing the active drug. [For Carzelesin, see: (a) Li et al., Cancer Res. 1992, 52:4904-4913; (b) van Tellingen et al., Cancer Res. 1998, 58:2410-2416; For KW-2189, see: (c) Kobayashi et al., Cancer Res. 1994, 54:2404-2410; (d) Nagamura et al., Chem. Pharm. Bull. 1995, 43:1530-1535; For other CBI carbamate pro-drugs: (e) Boger et al., Synthesis 1999, 1505-1509; (f) Wang et al., Bioorg. Med. Chem. 2006, 14:7854-7861; (g) Li et al., Tetrahedron Lett. 2009, 50:2932-2935; and (h) Wolfe et al., J. Med. Chem. 2012, 55:5878-5886.] Pro-drugs that use this protection and release strategy have been developed where the phenol release in vivo is coupled to features that might permit selective tumor cell delivery or cleavage, but surprisingly few pro-drug classes have been studied to date. [For glycosidic pro-drugs: (a) Tietze et al., Bioorg. Med. Chem. 2001, 9:1929-1939; (b) Tietze et al., Angew. Chem. Int. Ed. 2006, 45:6574-6577; and (c) Tietze et al., Bioorg. Med. Chem. 2008, 16:6312-6318. For reductively activated pro-drugs: (a) Hay et al., J. Med. Chem. 2003, 46:5533-5545; (b) Hay et al., Bioorg. Med. Chem. Lett. 1999, 9:2237-2242; (c) Tercel et al., Angew. Chem. Int. Ed. 2011, 50:2606-2609; (d) Townes et al., Med. Chem. Res. 2002, 11:248-253; and (e) Boger et al., J. Org. Chem. 1999, 64:8350-8362. For other pro-drugs, see: (a) Zhao et al., J. Med. Chem. 2012, 55:766-782; and (b) Pors et al., K.; Chem. Commun. 2011, 47:12062-12064.]
There are largely two main pro-drug strategies that have been examined for the duocarmycins. The most widely examined strategy utilizes an acylation of the phenol that can be cleaved hydrolytically or enzymatically. Many groups have examined such ester and carbamate pro-drugs [for Carzelesin, see: (a) Li et al., Cancer Res. 1992, 52:4904-4913; (b) van Tellingen et al., Cancer Res. 1998, 58:2410-2416; For KW-2189, see: (c) Kobayashi et al., Cancer Res. 1994, 54:2404-2410; (d) Nagamura et al., Chem. Pharm. Bull. 1995, 43:1530-1535; For other CBI carbamate pro-drugs: (e) Boger et al., Synthesis 1999, 1505-1509; (f) Wang et al., Bioorg. Med. Chem. 2006, 14:7854-7861; (g) Li et al., Tetrahedron Lett. 2009, 50:2932-2935; and (h) Wolfe et al., J. Med. Chem. 2012, 55:5878-5886] and two such derivatives, KW-2189 [Kobayashi et al., Cancer Res. 1994, 54:2404-2410] and carzelesin [Li et al., Cancer Res. 1992, 52:4904-4913], entered clinical trials. With the exception of a notable recent example [Wang et al., Bioorg. Med. Chem. 2006, 14:7854-7861], these have been traditionally designed to permit rapid free drug release upon in vivo administration and typically incorporate functionality to improve the drug physical properties (e.g., solubility).
The second but less extensively explored approach involves the development of functionality that can be reductively activated. [Wolkenberg et al., Chem. Rev. 2002, 102:2477-2495.] Past examples include Denny's nitro precursors to aryl amine variants of the phenol drugs [Hay et al., J. Med. Chem. 2003, 46:5533-5545], Lee's use of an ester that is subject to cleavage upon a tethered quinone reduction [Townes et al., Med. Chem. Res. 2002, 11:248-253], and a report of mitomycin-like quinone precursors that undergo an analogous o-spirocyclization upon reductive activation by the inventor and co-workers [Boger et al., J. Org. Chem. 1999, 64:8350-8362].
More recently, the inventor and co-workers introduced a set of N-acyl O-amino phenol pro-drugs that were designed to potentially take advantage of the hypoxic tumor environment and its increased concentration of reducing nucleophiles such as glutathione that can cleave an activated N—O bond (below) [Jin et al., J. Am. Chem. Soc. 2007,
129:15391-15397 and U.S. Pat. No. 8,377,981; and (b) Lajiness et al., J. Med. Chem. 2010, 53:7731-7738]. Just as significantly, intracellular concentrations of such reducing thiols are as much as 100-fold higher than plasma concentrations, suggesting that, unlike the more traditional carbamates, such reductively-activated pro-drugs can be subject to preferential intracellular release. Precedent for the behavior of such pro-drugs can be found in the natural product FR900482 and its related congeners [(a) Paz et al., J. Am. Chem. Soc. 1997, 119:5999-6005; and (b) Williams et al., Chem. Biol. 1997, 4:127-137; and Tepe et al., Tetrahedron 2002, 58:3553-3559] that contain hydroxylamine hemiketals and are irreversibly activated by reductive cleavage of an N—O bond.
In initial studies by the inventor and co-workers, a remarkable range of pro-drug stability and propensity for N—O bond cleavage was observed even with subtle variations in the electronic and steric environment around the weak N—O bond. Significantly, the in vivo evaluation of several such pro-drugs demonstrated that those that exhibit a balanced N—O stability/reactivity approach the potency and substantially exceed the efficacy of the free drug itself, suggesting that such pro-drugs can afford advantages related to their controlled or targeted release.